The invention relates to turbochargers used for boosting the performance of engines such as diesel and gasoline automotive engines and the like.
Turbochargers for diesel and gasoline automotive engines have conventionally employed oil-lubricated bearings. The turbocharger is connected to the engine oil system and engine oil is supplied into the bearing housing of the turbocharger to lubricate the bearings, and the oil is then discharged from the bearing housing and returned to the engine oil system.
Oil-lubricated turbochargers are prone to oil seal failures. If the oil seal on the turbine side of the turbocharger fails and allows oil to leak into the turbine, the oil ends up being discharged into the engine exhaust system, which can increase undesirable emissions from the engine.
The durability of an oil-lubricated turbocharger is highly dependent on the engine lubrication system. The now common use of low-viscosity oils and high engine oil temperatures to reduce friction adversely affects the stability and durability of the turbocharger bearing system, as well as the effectiveness of the oil seals. On the other hand, operation under very cold conditions can lead to a delay of the oil pressure reaching the turbocharger and can also result in turbo bearing durability problems.
Additionally, oil-lubricated turbochargers are restricted to being used in a particular horizontal orientation, since they depend on gravity for draining the oil from the bearings. As a result, the engine or vehicle designer does not have much freedom with respect to placement of the turbocharger.
These and other drawbacks of oil-lubricated turbochargers have led industry to seek to develop a practical turbocharger employing hydrodynamic air foil bearings. Foil bearings have been used in air cycle machines (turbomachines used in a cooling cycle where air is the working fluid) for the aerospace industry since approximately 1970. Such air cycle machines are used, for example, for supplying cooling air for electronics in military aircraft, and for supplying air for cabin climate control systems in commercial aircraft. It is relatively easy to adequately cool foil bearings in an air cycle machine because the machine operates at a relatively low temperature, typically a few hundred degrees Fahrenheit. However, prior to the present invention, it is believed no production turbochargers incorporating foil bearings have been developed, at least in part because difficulties have been encountered in adequately cooling the foil bearings. Without adequate cooling, foil bearings tend to have short useful lives.
Thus, while the benefits of incorporating foil air bearings into a turbocharger are well understood, significant technical challenges have to be overcome to develop a practical design. These challenges include developing an adequate bearing housing thermal design and cooling system to avoid overheating the foil coatings, providing adequate seals to prevent bearing contamination by engine oil and combustion products, and developing a high stiffness shaft design to avoid shaft instability at high speeds.
The present invention addresses the above needs and achieves other advantages.